This invention relates broadly to rotary injection molding of plastic components and, more particularly, pertains to rotary platens for supporting molds used in multi-shot injection molding of multi-colored or multi-material plastic parts.
In two-shot molding, the mold is turned or indexed by a rotating table platen. The platen is operated by a hydraulic motor or cylinder which rotates the mold 180xc2x0 for a two-shot cycle. The platen can also be indexed at other increments to accommodate more than two shots. This rotating platen is mounted on the moving platen of the molding machine and cooperates with a fixed metal mold mounted on a fixed platen forming part of the mounting frame in the molding machine.
To produce a two-shot plastic part, first one shot of material is injected into the mold, the mold then opens and the platen rotates the mold 180xc2x0, ejects the runner and the mold closes again. A second shot is then injected around the first shot to create a plastic part with two colors or materials. Simultaneously, the first shot is injected again. When the mold opens this time, both runners will eject along with the completed part. The mold will then rotate and close to repeat the cycle again. The rotary platen technique is frequently used because is permits parallel injection. This results in short cycle times, so that production is optimized.
Some prior art rotating platens used to support molds have been deficient in supporting molds because of their weight problems or move at too slow a speed for producing the expected results. Other rotating platens have been designed to rapidly turn the massive mold by using rack and pinion assemblies. However, such rotary platens rely upon expensive and, in many cases, complicated electronic controls, switches and sensors to slow and stop the momentum of the rotary platen, so that it may properly and efficiently cycle again.
Despite the many attempts of the prior art, there remains a need for a rotary platen assembly permitting multi-shot molding of plastic components in molds of large mass, which are driven, slowed and stopped in a simplified, easy to maintain and cost effective manner.
It is general object of the present invention to provide an injection molding machine having a rotary platen assembly for rotating a mold used in manufacturing multi-color or multi-layer products.
It is one object of the present invention to provide a rotary platen assembly having a shock absorbing arrangement for dampening the momentum of a platen-supported mold of large mass.
It is also an object of the present invention to provide a rotary platen assembly driven by a rack and pinion, and traveling along an arcuate path between a starting position and a finishing position defined by a stop arrangement.
It is another object of the present invention to provide a rotary platen assembly having a shorter cycle time than previous rotary platen designs.
In one aspect of the invention, a rotatable platen assembly is adapted to support a mold used in multi-shot plastic injection molding. The assembly includes a non-rotatable machine platen having a central bore formed therethrough. A non-rotatable base plate is connected to the machine platen, the base platen having a central opening aligned with the central bore of the machine platen. A rotatable support plate is adapted to retain a mold thereon. The platen is provided with a rotatable center axle passing through the central opening and the central bore, and a gear mounted for rotation on the axle. A rack system is engageable with the gear for rotating the mold supporting plate between a starting position and a finishing position. A shock absorber arrangement is mounted externally of the base plate for dampening the momentum of the rotating plate between the starting position and the finishing position. The base plate is provided with a pair of fixed stops spaced apart and defining the starting position and finishing position of the rotating platen travel. The rotating plate is provided with a rotating stop which travels in an arcuate path as the rotating plate rotates. The shock absorbing arrangement includes a pair of shock absorbers secured on respective mounting plates connected to the base plate. The rack system includes a hydraulic cylinder having an extendable and retractable piston rod connected to a rack engageable with the gear. The base plate is formed with a straight passageway or slot receiving the rack, and the arcuate path for guiding the rotating stop therein. A cooling water jacket surrounds the center axle, the water jacket having a radially extending end portion disposed in the base plate and the machine platen, and a cylindrical body portion located in the machine plate. The base plate, the cooling jacket, the center axle and the rotating platen are all formed with a plurality of circuitous cooling fluid channels. The periphery of the rotating plate is engageable with a series of spaced apart jibs anchored to the base plate. A pair of spaced apart electronic sensors is mounted on the base plate for verifying the position of the rotating plate. A first set of bearings is disposed in the base plate and encircles the center axle. A second set of bearings is disposed between the center axle and the water jacket. A lower portion of the center axle is provided with a set of vertically spaced O-rings disposed between an outer wall of the center axle and an inner wall of the body portion of the water jacket. The base plate is provided with a set of cam followers for holding the rack against the gear. The first stops are located 180xc2x0 apart and the arcuate path is preferably semi-circular.
The invention also contemplates a multi-shot injection molding apparatus having a rack system for moving a mold-supporting rotating plate between a starting position and a finishing position relative to a non-rotating structure. The improvement includes a shock absorbing arrangement associated with the non-rotating structure for cushioning the momentum of the rotating plate and its mold as the rotating plate approaches the finishing position. A non-rotating structure is provided with spaced apart stops along an arcuate path for defining the starting position and finishing position of the rotating plate. The rotating plate is provided with a rotating stop engageable with the spaced apart stops.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.